Black and white color cholesteric liquid crystal display

ABSTRACT

A cholesteric display may be formed, in some embodiments, using a single display element to produce multi-colors for display. A cholesteric material may be sandwiched between a pair of substrates, each associated with pairs of opposed electrodes that are arranged in general transversely to the optical axis of incident light. The first pair of electrodes produce one of two liquid crystal states and result in the reflection of light of a particular wavelength. Light of other wavelengths may be reflected when a second pair (or set) of opposed electrodes, arranged in general transversely, also to the optical axis of incident light, are biased appropriately. So does a third pair (or set) of electrodes. A black and white color display may be generated from a single display element by modulating the pitch length of the cholesteric material within each pairs (or sets).

BACKGROUND

[0001] This invention relates generally to liquid crystal displays andparticularly to cholesteric displays.

[0002] Commonly, liquid crystal material may be modulated to produce adisplay. Conventional liquid crystal displays commonly use twistednematic (tn) liquid crystal materials having a pair of states that maydifferentially pass or reflect incident light. While twisted nematicdisplays may be reflective or transmissive, cholesteric displays areusually reflective (but they may also be transmissive).

[0003] In cholesteric displays, the cholesteric material has very highoptical activity. Such liquid crystal material switches between areflective texture called the planar cholesteric texture and thetransparent configuration with the focal conic texture. The cholestericmolecules assume a helical configuration with the helical axisperpendicular to the surface of the substrates.

[0004] The cholesteric liquid crystal molecules, in response to anelectric field, align as planar texture with the optical axis,reflecting light of a particular wavelength. Generally, the maximumreflection in the planar cholesteric texture is at a wavelength directlyproportional to the material's pitch distance.

λ₀ =n·p (where p=pitch length, n=(n||+n ^(⊥))/2)

[0005] Conventionally, an electric field is applied in the direction ofthe optical axis in order to change the phase and the texture of thecholesteric material. However, these changes are generally in the formof the material either being reflective to the spectrum of light of agiven wavelength or not reflecting light at all.

[0006] Thus, a given completed cholesteric liquid crystal cell mayproduce reflected light with a specific color, such as red, green orblue, but not any combination of them. Therefore, the conventionalapproach is to provide separate cholesteric display elements for each ofthe three primary colors (e.g., red, green and blue). These separatedisplay elements may be stacked up one on top of the other in order togenerate the desired full color reflected light output. Alternatively,the three elements may be placed side by side each displaying the samecolor. The three different colors may be achieved using color filtermaterial.

[0007] The use of color filter material substantially reduces thedisplay brightness and increases the overall cost of the display.Similarly, the use of three separate cells in a stack effectivelytriples the cost of the display. Stacked elements may even reduce theoptical brightness of each display pixel.

[0008] Bistable reflective cholesteric displays are particularlyadvantageous for many portable applications. The bistable material isadvantageous because it may be placed in one of the two states that havedifferent optical properties. Once placed in either state, the materialstays in that state even when power is removed. Thus, a given displayedpixel may remain, without refresh, in a given state until it is desiredto change the optical information that is displayed. Being reflective innature, and hence avoiding the need of backlight plus avoiding the needfor refresh will substantially reduce power consumption of the displaysubsystem.

[0009] Thus, there is a need for displays, and particularly for bistablecholesteric displays, that can be fabricated at lower costs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a greatly enlarged, schematic cross-sectional view ofone embodiment of the present invention;

[0011]FIG. 2 is a partial, greatly enlarged, top plan view of thestructure shown in FIG. 1 in accordance with one embodiment of thepresent invention;

[0012]FIG. 3 is a partial schematic depiction of the embodiment shown inFIG. 1;

[0013]FIG. 4 is a diagram showing a bistable cholesteric display in anactive matrix display arrangement, in accordance with one embodiment ofthe present invention; and

[0014]FIG. 5 is a bistable cholesteric display cell in a passive matrixdisplay arrangement, in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION

[0015] Referring to FIG. 1, a cholesteric display may include a bistablecholesteric material 16 in one embodiment of the present invention. Thematerial 16 is sandwiched between two substrates 12 and 24. Thesubstrate 24 is advantageously substantially transparent and mayconventionally be glass with an absorbing undercoating such as carbonblack. The substrate 24 may or may not be transparent. The substrate 24may be made of a variety of materials. The substrates 12 and 24, in oneembodiment, may include transparent electrodes 14 and 22. Thetransparent electrodes 14 and 22, in one embodiment, may be made ofindium tin oxide (ITO).

[0016] Sandwiched between the substrates 12 and 24 is a sidewayselectrode 26 b and an opposed sideways electrode 26 c which, in turn, isalso opposed to a sideways electrode 26 d. Between the electrode 26 andthe substrate 24 is a material 20. In an active matrix embodiment, thematerial 20 may be a thin film transistor or other active element todrive the actual display. In a passive matrix embodiment, the material20 may be a row or column contact or electrode.

[0017] A display 10, shown in FIG. 2, may be formed out of a pluralityof pixels 40 arranged in a grid work array. Each pixel 40, such as apixel 42, may be divided into three or more subpixels 42 a, 42 b, and 42c. In one embodiment, each subpixel 42 may be responsible for generatinglight of a different wavelength. Thus, each pixel 40 may produce threedifferent wavelengths of light, such as red, green, and bluewavelengths.

[0018] Each subpixel 42 may include two sets of opposed transverseelectrodes 26. For example, the subpixel 42 a may have an opposedelectrode pair 26 b and 26 c and an opposed electrode pair 26 a and 26e. Thus, the pixel 40 e is divided into three subpixels 42 so as to haveapproximately the same area in each subpixel 42 in one embodiment of theinvention. In some cases, the electrodes 26 c, 26 f, and 26 e may becommon between two different subpixels. For example, the electrode 26 cis an electrode for the subpixel 42 a and the subpixel 42 b in oneembodiment of the present invention.

[0019] Referring to FIG. 3, the electrodes 14 and 22 apply an electricfield along the optical axis O of the display 10. The optical axis O isaligned with the direction of incident light “L”. The light L, directedtoward the upper surface of the substrate 12, passes through the uppersurface and the electrode 14 and is reflected (or not) by thecholesteric material 16, as indicated by the light beam R, to producethe perceived image. Since the light arrives at and is reflected fromthe top upper surface, the optical axis O is oriented generallytransversely to the substrates 12 and 24.

[0020] In conventional fashion, the electric field developed by theelectrodes 14 and 22 may cause the bistable cholesteric material 16 totransition between the reflective planar cholesteric texture and thetransparent, focal conic texture. The procedures for applying potentialsfor causing these transitions to occur are well known in the art.

[0021] In general, an electric field may be applied by an alternatingcurrent voltage source 30 that is electrically coupled to the electrodes14 and 22. When the cholesteric material 16 is in its transparenttexture, in some embodiments, the lower substrate 24 becomes visible.When the material 16 is in its planar cholesteric texture, light of agiven wavelength is reflected. That given wavelength is generallydetermined by the helical pitch of the material 16. In conventionalcholesteric displays, this pitch is defined and is fixed. Thus, inconventional cholesteric displays each display element either providesone reflected color or is transparent, displaying the color of thesubstrate 22.

[0022] In accordance with one embodiment of the present invention, theelectrodes 26 apply an electric field transversely to the optical axisO. In one embodiment, this transverse electric field may be applied fromflat planar electrodes 26 arranged generally transversely to theelectrodes 14 and 22.

[0023] The electrodes 26 may be coupled to their own separate potential32. The electrodes 26 need not be, but may be transparent.

[0024] The electrodes 26 allow the pitch set by the electrodes 14 and 22to be varied. In one embodiment of the present invention, the electrodes26 enable the fixed pitch to be varied between three different pitches.Each of the different pitches, associated with a given potential on theelectrodes 26, may produce one of three different light colors. In oneembodiment, for example, red, green and blue light may be selectivelyproduced from a single display element 10.

[0025] In some embodiments, curved surface electrodes, such asdish-shaped electrodes having axes generally transverse to the opticalaxis O can be used. The sides of the curved surface of the dish-shapedelectrode provides the sideways electric field (from 360°) transverse tothe electric field aligned with the optical axis O.

[0026] Liquid crystals have dipoles that align in an applied electricfield. This property allows an electric field transverse to the opticalaxis to modify the pitch length of the material.

[0027] To generate the black color, the pitches of the material in eachpixel 40 may be calibrated to not reflect any visible light and, thus,the pixel 40 becomes dark or black after addressed.

[0028] In order to generate light for a black and white display, forexample, the helix of the material within each of the subpixels 42 maybe appropriately altered to separately produce red, green, and bluelight at the same time. The complementary reflectance of these threecolors renders a pixel 40 white in color as a whole. Thus, any color maybe produced by operating one of the three subpixels 42 and the colorwhite may be produced by operating all of the subpixels 42. Conversely,in one embodiment, when none of the subpixels are reflective, the pixel40 appears to be dark or black.

[0029] The geometry of the subpixels 42 is subject to considerablevariation. In general, it is only desirable that the subpixels 42 havesimilar areas in one embodiment of the present invention.

[0030] Through the use of electrodes 26, a multi-colored display pixelmay be produced with only a single cholesteric display element. As aresult, substantial cost savings may be achieved by avoiding the needfor three different display elements that are either laterally displacedfrom one another or stacked one atop the other. Moreover, when threedisplay elements are utilized in a laterally displaced arrangement,color filter arrays are generally needed and color filter arrays wouldsignificantly increase the cost of the display.

[0031] In one embodiment of the present invention, the material 16, whenexposed to the electric field aligned with the optical axis O, reflectslight in a central or intermediate wavelength of approximately 560nanometers. Then the pitch may be changed using the electric fieldapplied through electrodes 26 to either increase the reflectedwavelength, for example to 670 nanometers, or to decrease the reflectedwavelength, for example to 450 nanometers. This basically changed thereflected colors of the cell or element.

[0032] Other variations may be utilized, as well. In some embodiments itmay be efficient to provide the color red or the color blue when theelectrodes 26 are not operating and then to tune the pitch to adjust thereflected wavelength upwardly or downwardly using the electrodes 26. Atransmissive mode may also be used. In some embodiments, pitch changesmay be used to selectively reflect and/or transmit different wavelengthsof the spectrum, including those of the infrared range.

[0033] Referring to FIG. 4, in one embodiment of the present invention,an active matrix display may be implemented. In such case, the material20 may constitute a thin film transistor or other active element. In oneembodiment, the gate 22′ of the thin film transistor may be coupled to aline 36, that is in turn coupled to the electrode 22. At the same time,the source of the transistor 20 is coupled via line 38 to the electrode14. The drain 22″ may be coupled via a line 40 to an appropriate groundconnection in one embodiment of the present invention. An externalstorage capacitor 34 may be provided in some embodiments.

[0034] Similarly, in a passive matrix display embodiment, shown in FIG.5, the electrode 14 may be coupled to a column potential and theelectrode 22 may be coupled to a row potential. In such passive matrixaddressing case, a thin film transistor is not needed to provideelectrical addressing with row and column potentials on the material 20.

[0035] While the present invention has been described with respect to alimited number of embodiments, those skilled in the art will appreciatenumerous modifications and variations therefrom. It is intended that theappended claims cover all such modifications and variations as fallwithin the true spirit and scope of this present invention.

What is claimed is:
 1. A cholesteric display comprising: a pair ofopposed substrates; a cholesteric material between said substrates; afirst pair of electrodes aligned generally parallel to said substrates;and a plurality of second electrodes arranged generally transversely tosaid substrates to define a pixel including three subpixels.
 2. Thedisplay of claim 1 wherein said cholesteric material is a bistablecholesteric material.
 3. The display of claim 1 wherein said secondelectrodes enable the display to produce light of variable wavelengths.4. The display of claim 1 wherein said cholesteric material selectivelyproduces light of one of a plurality of possible wavelengths.
 5. Thedisplay of claim 1 wherein said first pair of electrodes are formed onsaid pair of opposed substrates.
 6. The display of claim 5 wherein saidfirst pair of electrodes are formed of transparent conducting material.7. The display of claim 1 wherein said second electrodes are arranged tochange the pitch of said cholesteric material.
 8. The display of claim 1wherein said display is a reflective display.
 9. The display of claim 1wherein when said first pair of electrodes are operational, said displayproduces a first wavelength of light and when said second electrodes areoperational, said display produces two additional wavelengths of light,including a wavelength longer than said first wavelength and awavelength shorter than said first wavelength.
 10. The display of claim1 wherein said display does not include a color filter.
 11. The displayof claim 1 wherein each of said subpixels are approximately the samesize.
 12. The display of claim 1 wherein each of said subpixels includesat least two electrodes of said second electrodes.
 13. A methodcomprising: exposing a cholesteric material to light along an opticalaxis; generating an electric field generally parallel to said opticalaxis; and forming at least three subpixels producing differentwavelengths by generating an electric field generally transverse to saidoptical axis.
 14. The method of claim 13 including causing saidcholesteric material to reflect light of more than one wavelength. 15.The method of claim 14 including causing said cholesteric material toselectively produce light of one of three distinct wavelengths.
 16. Themethod of claim 13 including producing light of different wavelengthsusing said second electric field without the use of a color filterarray.
 17. The method of claim 13 including exposing a cholestericmaterial to said first and second electric fields.
 18. The method ofclaim 13 including changing the pitch of said material using said secondelectric field.
 19. The method of claim 13 including actively drivingsaid cholesteric material.
 20. The method of claim 13 includingpassively operating said cholesteric material.
 21. The method of claim13 including causing said subpixels to selectively produce light in thered, green and blue wavelengths.
 22. The method of claim 13 includingexposing a cholesteric material to a first electric field to switch thecholesteric material between transparent and reflective textures andexposing said cholesteric material to a second electric field transverseto said first electric field to change the pitch of said cholestericmaterial.
 23. The method of claim 13 including operating all threesubpixels to generate white light.
 24. A cholesteric element comprising:a cholesteric material; a pair of substrates surrounding saidcholesteric material; and a device to cause said cholesteric material toselectively generate light in different wavelengths and to generatewhite light.
 25. The element of claim 21 wherein said device includes afirst pair of electrodes aligned generally parallel to said substratesand a plurality of second electrodes arranged generally transversely tosaid substrates.
 26. The element of claim 21 wherein said secondelectrodes change the pitch of said cholesteric material to selectivelyalter wavelengths of the spectrum.
 27. The element of claim 21 whereinsaid cholesteric material is a bistable cholesteric material.
 28. Theelement of claim 21 wherein said element is a display.
 29. The elementof claim 25 wherein said display is an active matrix display.
 30. Theelement of claim 25 wherein said element is a passive matrix display.